1. Field of the Invention
The invention relates to polytetrafluoroethylene (PTFE) micropowder compositions containing mixtures of PTFE micropowders, and fluoroelastomer compositions that contain the micropowder compositions. The invention further relates to mixing, milling, molding or extrusion processes that include mixing, milling, molding or extruding tetrafluoroethylene/propylene elastomers in the presence of the PTFE micropowder compositions and may include adding one or more additives to a fluoroelastomer composition as a mixture with the PTFE micropowder compositions.
2. Description of the Related Art
Polytetrafluoroethylene (PTFE) micropowders may be used as additives in fluoroelastomer compositions to provide improved abrasion resistance, reduced coefficient of friction, and improved surface characteristics in molded and/or extruded parts. High molecular weight PTFE has been used in elastomeric compositions as a reinforcing additive to improve properties such as tear strength. High molecular weight PTFE may be added to the elastomer or fluoroelastomer during compounding and/or extrusion. It is thought that high molecular weight PTFE fibrillates during mixing and/or extruding to form a network of nodes and fibers that reinforces the elastomer or fluoroelastomer matrix.
An undesired side effect caused by the addition of high molecular weight PTFE to an elastomeric composition may include the formation of surface blemishes during extrusion and/or molding. In addition, unacceptably high hardness and/or modulus may result in cured vulcanizates of PTFE-containing fluoroelastomer or elastomer compositions. Thus, cosmetic defects caused by the PTFE fibril network have limited the use of such compositions to applications where surface appearance is not critical or important.
A reduction in the molecular weight of the PTFE or the use of high molecular weight tetrafluoroethylene (TFE) copolymers comprising tetrafluoroethylene monomer units copolymerized with monomers such as hexafluoropropylene or perfluoro(propylvinylether) may form compositions that can be extruded and/or molded with substantially less surface blemishing (see “Reinforcement with Fluoroplastic Additives”, R. A. Morgan, C. W. Stewart, E. W. Thomas and W. M. Stahl, in Rubber World May 1991). High molecular weight copolymers of tetrafluoroethylene and hexafluoropropylene are commercially available (e.g., Teflon® MP1500, DuPont) and may be processed smoothly with elastomer or fluoroelastomer polymer matrices. PTFE-based compositions containing high molecular weight TFE copolymers may provide lowered surface blemishing and easier incorporation of fluoropolymer additives into a fluoroelastomer matrix during extrusion or molding. However, lower molecular weight PTFE or fluoroolefin copolymers may exhibit less fibrillation and consequently the resulting compositions may provide reduced tensile strength and less than satisfactory tear properties in comparison to compositions containing higher molecular weight PTFEs.
Lower molecular weight PTFEs may be formed by irradiating high molecular weight PTFEs. Radiation treatment may lead to bond scission and the formation of PTFE having a reduced average molecular weight. Conventionally it has been thought that lower molecular weight PTFEs are not capable of improving the physical properties of fluoroelastomers or other elastomers because lower molecular weight PTFE materials are not capable of providing sufficient fibrillation and hence may not form extensive reinforcing networks. Further, extruding a fluoroelastomer with a lower molecular weight PTFE may cause the PTFE to take the form of continuous long fiber networks, short fibers, elongated platelets, or ribbons formed by the shear stresses exerted upon the elastomeric matrix during processing. This ribbon and/or fibrous form network may not provide the desirable reinforcing properties of fibrillated high molecular weight PTFEs.
Micropowders may be added to elastomeric compositions to enhance the surface and bulk properties of molded parts such as surface lubricity, tear strength and flex life. These micropowders may include PTFE materials which are usually present in amounts no greater than an amount that causes a reduction in the elastic characteristics of the composition.
The flex properties and compressability of elastomer compounds is especially important in applications such as seals that operate under extreme pressure or temperature conditions. Loss of compressability or any tendency towards deformation under harsh conditions such as exposure to materials which may penetrate or otherwise degrade the matrix from which the seal is molded, are often encountered in industries such as the chemical industry and in oil processing.
Fluoroelastomer compositions that contain semi-crystalline fillers may provide improved mechanical properties when the filler is added as a nano-powder. The form of the filler (e.g., PTFE powder) is important in determining whether the resultant composition will provide the desired properties (see “Technoflon Fluoroelastomers and Perfluoroelastomers: The Right Choice for Oil Field Applications,” S. Arrigoni, L. Colombo, A. Minutillo, and G. Sanvito, Oil Field Engineering with Polymers, London, England November 2003, pp. 249-257). Mixing a perfluoropolymer with a fluoroelastomer to form a coagulate prior to extrusion or molding provides, upon extrusion, a composite matrix containing islands of the perfluoroolefin material encapsulated in a matrix of the fluoroelastomer. Such compositions have mainly used perfluoroolefin micropowders such as PTFE of narrow molecular weight range as a reinforcement and/or filler.
Also important in determining whether or not a particular fluoroelastomer/PTFE composition may provide improved physical characteristics is whether the presence of other fillers such as carbon-based fillers which may be necessary to impart other desirable properties to the resulting elastomeric composition will negatively affect the properties of the composition.